1. Field of the Invention
The present invention relates to a method for sorting monolithic ceramic capacitors, and more specifically, it relates to an improvement thereof which increases the efficiency of the sorting process and increases the reliability of the sorting results.
2. Description of the Related Art
Monolithic ceramic capacitors may encounter a problem in which defects, such as cavities in a ceramic dielectric substance after burning, are produced when foreign materials are mixed or coagulation substances are produced in a ceramic dielectric substance during manufacturing. Since these defects cause the deterioration of the insulation resistance of a monolithic ceramic capacitor, products having such defects should be sorted out to be eliminated at least before shipping.
In order to sort out defective products for elimination as mentioned above, an insulation resistance of a monolithic ceramic capacitor is generally measured in a manufacturing process of the monolithic ceramic capacitor.
However, the method for judging the presence of a defect by measurement of an insulation resistance as described above is essentially to detect the presence of the defect indirectly. Therefore, when the defect is minute, it is almost impossible to detect it using the indirect method of evaluation of measuring insulation resistance. Thus, in effect, the defect cannot be detected by the evaluation of an insulation resistance. In other words, a capacitor may have a minute defect but still have normal insulation resistance.
However, even a minute defect will cause the insulation resistance of a monolithic ceramic capacitor to deteriorate over time. This is, particularly true of monolithic ceramic capacitors having thin dielectric layers. Therefore, a highly reliable sorting method enabling defects to be detected is required especially as the dielectric layers of the capacitors become thinner.
Recently, there are uses of a monolithic ceramic capacitor requiring long-term reliability, such as military uses, space-technological uses, and automotive uses. In monolithic ceramic capacitors for these uses, high reliability is required in the process for sorting the results of testing. In conjunction with this matter, the following US Military Specification are known as methods for elimination of defective items and confirmation of qualities with a high degree of reliability.
(1) xe2x80x9cMIL-C-39014E, Item 4.7.2.2, Thermal Shock and Voltage Conditioningxe2x80x9d. Double the rated voltage is applied for 96 hours at the maximum working temperature of the capacitor to actualize defects. After that, an insulation resistance is measured at room temperature to detect defects by the degree of deterioration of the insulation resistance. This method is a kind of burn-in.
(2) xe2x80x9cMIL-C-55681C, Item 3.9, Insulation Resistance b. at 125xc2x0 C.xe2x80x9d. An insulation resistance is measured at 125xc2x0 C. through the application of the rated voltage to confirm that the resistance is not less than the prescribed value.
However, any of the above-mentioned defect detecting methods according to the US Military Specification present problems.
In the above-mentioned method (1), conditions applied to a capacitor are severe so that a reliable evaluation result can be obtained. However, the method is not efficient as it takes at least 96 hours to obtain the evaluation result. Therefore, when numerous monolithic ceramic capacitors should be evaluated on a 100 percent basis, the method is not practical.
Then, in the above-mentioned method (2), although the evaluation can be completed in a comparatively short time, the reliability of the evaluation result is not satisfactory in comparison with the above-mentioned method (1).
Accordingly, it is an object of the present invention to provide a sorting method for monolithic ceramic capacitors in which the efficiency is improved and a reliable sorting result is obtained.
To put it simply, the object of the present invention is to solve the above-described technical problems by performing both the methods (1) and (2), which are US Military Specification.
In further detail, a method for sorting of monolithic ceramic capacitors according to the present invention comprises the steps of applying not less than double a rated voltage at a maximum working temperature to a monolithic ceramic capacitor to be sorted as a burn-in process step; and measuring insulation resistances while applying not less than the rated voltage at a temperature of not less than 70xc2x0 C. to the same monolithic ceramic capacitor as a high temperature insulation resistance measuring process step. As a result, monolithic ceramic capacitors having abnormal insulation resistances can be detected.
In the invention, it may be preferable that the burn-in process be performed in advance while the high temperature insulation resistance measuring process be performed afterward.
In this case, the high temperature insulation resistance measuring process may be performed after the burn-in process without measuring an insulation resistance at room temperature, or the method may further comprise a step of measuring an insulation resistance at room temperature while applying the rated voltage to the monolithic ceramic capacitor after the burn-in process and before the high temperature insulation resistance measuring process.
In the invention, the burn-in process and the high temperature insulation resistance measuring process may be preferably performed as continuous processes.
The direction of applied voltage to the monolithic ceramic capacitor in the burn-in process may preferably agree with the direction of applied voltage to the monolithic ceramic capacitor in the high temperature insulation resistance measuring process.